The present invention relates to a drying method and a drying equipment for drying objects to be dried, such as cleaned wafers of semiconductors, substrates for liquid crystal displays, substrates for recording disks, etc.
A vapor dryer 70 shown in FIG. 10, for example, is known equipment for drying to-be-dried objects, such as rinsed semiconductor wafers (hereinafter referred to simply as wafers). The vapor dryer 70 comprises a body 71, carrier 72 for carrying a wafer 3, receiver tray 73, etc. The body 71 has a dual structure including a vapor bath 74 as a treatment bath containing an organic solvent Y and an outer chamber 75 surrounding the bath 74. Isopropyl alcohol (hereinafter referred to as IPA) is an example of the organic solvent Y. A heat block 77 is provided between a bottom portion 76 of the vapor bath 74 and the outer chamber 75. As the organic solvent Y is heated by the heat block 77, it evaporates and forms a vapor V.
The wafer 3 to be dried is washed with a cleaning fluid, such as water, in a cleaning process before it is put into the vapor bath 74. Immediately after it is put into the bath 74, therefore, the wafer 3 has the vapor on its surface, and its temperature is relatively low. The wafer 3 having the fluid thereon is exposed to the vapor V while held by means of a carrier (not shown) and is put into the vapor bath 74. In the bath 74, the organic solvent vapor V condenses on the surface of the wafer 3, whereby the organic solvent replaces the vapor or water on the wafer surface. The vapor or water flows down from the wafer surface and is recovered by the receiver tray 73. As the organic solvent on the surface of the wafer 3 evaporates, the wafer 3 is dried in a short period of time. As the cleaning fluid (water) flows down from the wafer surface, foreign matter or particles on the wafer surface are washed away by the fluid. If the cleaning fluid only flows down, however, the particles cannot be thoroughly removed from the wafer surface, in some cases.
The wafer 3 is washed with the cleaning fluid such as water is at a relatively low temperature immediately after it is put into the vapor bath 74. Accordingly, the vapor V condenses on both the mirror and non-mirror surfaces of the wafer 3 at the same time. This condensation continues until the temperature of the wafer 3 becomes equal to that of the vapor V. Hereinafter, this condensation will be referred to as "primary condensation."
The temperature of the wafer 3 in the vapor bath 74 gradually increases, and finally reaches a heat balance point, which is substantially equal to the temperature of the vapor V. When this condition is established, latent heat of the vapor V condensed on one side (e.g., mirror surface) of the wafer 3 is absorbed by the wafer 3. Since the wafer 3 is kept thermally balanced in the vapor V, heat in the same quantity as the latent heat is transmitted to the reverse surface of the wafer 3. On the reverse surface of the wafer 3, therefore, the heat of vaporization is applied to the vapor V going to condense or having condensed. Thus, on the reverse side of the wafer 3, the vapor V fails to condense, so that a dry condition is maintained. This condensation will hereinafter be referred to as "thermally-balanced condensation."
In the vapor bath 74, the vapor V exists on both the mirror and non-mirror surfaces of the wafer 3. In the case of the aforesaid "thermally-balanced condensation," therefore, if the vapor V condenses on either of the mirror and non-mirror surfaces, the other surface is inevitably dried up. Thus, the mirror and non-mirror surfaces of the wafer 3 are brought to a very unstable condition such that they are liable to cause the vapor V to condense or be dried up. Accordingly, the mirror and non-mirror surfaces of the wafer 3 alternately repeat vapor condensation and drying, so that the following problems are aroused.
The organic solvent Y in the vapor bath 74 heated by the heat block 77 produces the vapor V. If the vapor V condenses on the wafer surface as the wafer 3 is put into the vapor bath 74, however, the volume of the vapor V is reduced drastically. Since the heat block 77 continues to heat the organic solvent Y, the solvent continues to produce the vapor V. However, it sometimes takes, for example, tens of seconds for the whole wafer 3 to be covered again with the vapor V. In this case, the temperature of the wafer 3 reaches the temperature level of the vapor V so that the aforesaid condition of "thermally-balanced condensation" is established before the whole wafer 3 is covered with the vapor V. As a result, dry portions develop on either the mirror or non-mirror surface of the wafer 3, and particles on those portions may remain without being removed, in some cases. Before the wafer 3 is entirely covered again with the vapor V, moreover, it is exposed to air, and therefore, is subjected to air-drying. Inevitably, therefore, natural oxide films or spots, called watermarks, are formed along the respective contours of drops of the vapor having so far been on the wafer 3.